1. Field of the Invention
This invention relates to spectrometry, and more particularly to differential spectrometry using discrete optical filters.
2. Description of the Related Art
Optical interference filters are used for a wide variety of applications, particularly in the field of spectrometry. For example, a narrow band optical filter may be employed in a system that is designed to detect the presence of a particular chemical compound in the atmosphere. The filter is designed to transmit only the light that falls within the absorption band of the desired chemical compound, thereby masking other absorption bands found in the atmosphere.
Interference filters are described in H. A. Macleod, Thin-Film Optical Filters, Second Edition, Macmillan Publishing Co., New York (1986), pages 234-313. They typically comprise a quarter-wave (QW) stack that is deposited on an optically transmissive substrate by vacuum evaporation. A QW stack consists of alternating high and low refractive index material layers whose optical thicknesses (refractive index times physical thickness) are tailored to produce a 90 degree phase shift in the light that is transmitted through each layer at a design wavelength. The QW stack is an efficient reflector of light over a wavelength range-(stopband) 5 that is centered about the design wavelength, as illustrated in FIG. 1a. The width of the stopband 5 is dependent upon the ratio of the high and low refractive indices of the alternating material layers.
On either side of the stopband 5 are regions of high transmittance 6 with moderate to severe "ripples" in the shape of the transmittance curve. An edge filter is constructed by using either side of the transition between the high reflectance stopband 5 and the high transmission regions 6. A cut-on or long-wavelength pass (LWP) edge filter is constructed by modifying the basic QW design to minimize the ripple on the long wavelength side of the stopband 5, as illustrated in FIG. 1b. Alternatively, a cut-off or short-wavelength pass (SWP) edge filter is constructed by modifying the QW design to minimize the ripple on the short wavelength side of the stopband 5, as illustrated in FIG. 1c. The wavelength at which the edge (or boundary) of either type of filter appears is controlled by controlling the wavelength at which the center of the stopband 5 appears.
An extension of the use of QW stacks is to combine a LWP filter at one design wavelength with a SWP filter at a longer wavelength to produce a bandpass (BP) filter with the transmittance characteristics shown in FIG. 1d. This type of filter transmits light whose wavelength falls between .lambda..sub.1 and .lambda..sub.2. The edges 8 are defined by the LWP cut-on and SWP cut-off wavelengths. Light with a wavelength that falls outside of the edges 8 is reflected by the QW stack.
A problem associated with optical interference filters is that the peak optical transmittance at the design wavelength decreases as the filter's bandwidth is made narrower. The reduced optical transmittance reduces the signal-to-noise ratio to a level which may be unacceptable.